# Wavefronts and phase velocity faster than $c$

Lets assume we have parallel wavefronts in a glass of water:

and we put an inclined rod on the water surface:

related to a very small inclining, Vy velocity is greater or much greater then Vx (Vy means, wavefronts' contact-making speed on the rod)

Now lets assume environment is space and the waves are of electro-magnetic ones: would Vx be smaller than c? Would Vy and Vx be equal? I dont think Vy would be greater than c. What do you think?

Assume inclined rod is just a metal rod and photo-electric effect is intact.

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## 1 Answer

Yes, $v_y$ can be greater than $c$, and in fact it could be as large as you want if the make the angle small enough. However nothing, i.e. no signal, is being transmitted at that velocity so it doesn't cause any faster than light travel issues.

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If you have a flat enough blade you can even do this with a knife edge - ie. make the point on Vy move 'up' faster than c –  Martin Beckett Sep 14 '12 at 17:30